Rapid Gas Generating Pyrotechnical Composition and Method for Obtaining Same

ABSTRACT

The subject-matter of the present invention is:
         pyrotechnic compositions generating gases, whereof at least 95% by mass of the charges are constituted by constitutive ingredients hereinbelow:   a reducing charge in the form of at least one organic nitrogen compound;   an oxidizing charge in the form of at least one basic metal nitrate; and   less than 30% by mass of a second oxidizing charge in the form of at least one alkaline metal perchlorate; and
 
which are obtained via a dry roller compacting step of a pulverulent mixture essentially constituted by said powder ingredients; and also
   the method for obtaining such compositions.       

     The compositions of the invention have interesting combustion speeds.

The present invention relates to pyrotechnic gas generation, especiallyfor inflating cushions used in systems for protecting occupants of anautomobile. The present invention more precisely relates to pyrotechniccompositions known as cold pyrotechnic compositions rapidly generatingclean and non-toxic gases at temperatures acceptable for automobilesafety (temperatures qualified as low, that is, under 2200 K). Thepresent invention also relates to a method for producing suchpyrotechnic compositions.

Within the scope of automobile safety, the pyrotechnic compositionsemployed in gas generators must provided the quantity of gas necessaryfor putting in place the inflatable cushion in an extremely short time,typically between 10 and 40 milliseconds. Also, the gases generated mustbe clean, that is exempt from solid particles (likely to constitute hotpoints which might damage the wall of said cushion) and non-toxic, thatis having low content of carbon monoxide, nitrogen oxides andchlorinated products.

Various types of pyrotechnic compositions have already been proposed todate.

Currently, those pyrotechnic compositions appearing to offer the bestcompromise in terms of gas temperature, gas yield, rate of particlesemitted and toxicity contain, as principal ingredients, guanidinenitrate (GN) and basic copper nitrate (BCN). U.S. Pat. No. 5,608,183describes such compositions, obtained by wet method.

These compositions do however have the disadvantage of having relativelyslow combustion speeds, less than or equal to 20 mm/s at 20 MPa andbeing difficult to ignite. Similarly, they are very difficult to use inhybrid and side generators which need very brief operating time, between10 and 20 milliseconds.

It has been proposed, according to the prior art, to add perchlorates tosuch pyrotechnic compositions based on guanidine nitrate (GN) and basiccopper nitrate (BCN):

-   -   in patent application EP 1 526 121, the addition of perchlorate        is described (especially potassium perchlorate), at a low rate        (less than 5% by mass), to improve ignition of said compositions        and decrease emission of nitrogen oxides;        -   in U.S. Pat. No. 6,893,517, the addition of perchlorate is            described (especially potassium perchlorate), at relatively            high rates (between 30 and 45% by mass), to boost the            combustion speed of said compositions. Such rates of            perchlorate cause high combustion temperatures, of around            2400 K. The pyrotechnic compositions in question can no            longer be considered as cold, with respect to the            non-aggression of inflatable cushions by the combustion            gases.

In reference to the technical problem of a speed increase in combustionof pyrotechnic compositions based on guanidine nitrate (GN) and basiccopper nitrate (BCN), the addition of perchlorate is therefore not asatisfactory solution per se.

The roller compacting and dry granulation method is also known,implemented in different contexts, for treating powders, generallymixtures of powders. Such mixtures of powders, coming from a dry powdermixer, are transported, for example by a metering screw, to supply acylinder compacter. Such a compacter is composed of two rotatingcylinders, placed in rotation at a defined speed, in the oppositedirection. The powder mixture is pushed by the metering screw betweensaid two cylinders. A known effort is applied to the cylinders.Accordingly, the material passing between them, at a given rate, iscompacted in the form of a flat plate. Such an operation places a strongrate of compression and shearing on the mixture which improves thecloseness between the constituents. The compacted product, originatingfrom the compacter is then broken and forced through a grater togenerate granules. Such granules generally turn out to be easier tohandle than the starting powders. A single device generally successivelymanages both compacting and granulation.

Within the scope of the present invention, the inventor has shown, inreference to the technical problem of the increase in combustion speed,that the major interest is to carry out dry roller compacting in thecontext of the making of pyrotechnic compositions of guanidine nitrateand basic copper nitrate type. A real synergy has surprisingly beenobserved: the positive effect (on the combustion speed) due tointervention of a limited quantity of perchlorate (without harmfuleffect on the combustion temperature) is potentialised by carrying outdry roller compacting; use of which has per se no substantial effect inthe absence of perchlorate. In support of this affirmation, comparativedata hereinbelow can be supplied on combustion speeds at 20 MPa:

GN+BCN≦20 mm/s

GN+BCN+compacting 20-22 mm/s

GN+BCN+KClO₄ (example 4 hereinbelow) 32 mm/s

GN+BCN+KClO₄+compacting (example 5 hereinbelow) 38.2 mm/s.

According to its first object, the present invention therefore relatesto pyrotechnic compositions generating gases which combine twocharacteristics. Said pyrotechnic gas compositions:

-   -   include, as principal constitutive (active) ingredients:    -   a reducing charge in the form of at least one organic nitrogen        compound;    -   an oxidizing charge in the form of at least one basic metal        nitrate; and    -   less than 30% by mass of a second oxidizing charge in the form        of at least one alkaline metal perchlorate,

said charges representing at least 95% by mass (generally at least 98 bymass) of the charges present; and

-   -   are obtained via a dry roller compacting step of a pulverulent        mixture essentially made up of said powder ingredients.

The pyrotechnic compositions of the invention, including a specificreducing charge (in the form of at least one organic nitrogen compound)and a specific oxidizing charge (in the form of at least one basic metalnitrate), include a limited quantity of a specific second oxidizingcharge (in the form of at least one alkaline metal perchlorate) and areobtained on completion of a method including a dry roller compactingstep of the pulverulent mixture including said specific reducing andoxidizing charges.

Dry roller compacting is carried out, as known per se, in a cylindercompacter, generally at a compacting pressure of between 10⁸ and 6.10⁸Pa.

The method for producing the pyrotechnic compositions of the inventionwhich characteristically includes a dry roller compacting step, isdescribed in detail further on in the present text.

It can be carried out according to different variants (with acharacteristic step of “simple” roller compacting followed by at leastone additional step, with a characteristic step of roller compactingcoupled with a forming step . . . ) and the pyrotechnic compositions ofthe invention therefore exist in different forms.

In fact:

-   -   on completion of the dry roller compacting coupled with forming        (by using at least one compacting cylinder, whereof the external        surface has alveoli), plates with relief patterns are obtained        which can be broken to directly obtain shaped pyrotechnic        objects;    -   on completion of dry roller compacting followed by granulation,        granules are obtained;    -   on completion of dry roller compacting followed by granulation        then pelleting (dry compression), pellets are obtained;    -   on completion of dry roller compacting followed by granulation        and then mixing the granules obtained with an extrudable binder        and extrusion of said binder loaded with said granules, extruded        monolithic blocks are obtained (loaded with said granules).

The pyrotechnic compositions of the invention are therefore likely toexist in the form of:

-   -   shaped objects, originating directly from roller compacting        (coupled with forming);    -   granules;    -   pellets; and    -   extruded monolithic blocks (loaded with granules).

In a non-limiting manner it can be indicated here that:

-   -   granules according to the invention generally present        granulometry (median diameter) of between 200 and 800 μm (as        well as apparent volumic mass between 0.8 and 1.2 cm³/g);    -   pellets according to the invention generally have a thickness        between 1 and 3 mm; and    -   within the extruded monolithic blocks there are granules in a        dry binder (gel).

It can also be pointed out here that the followings especially form partof the first object of the invention:

-   -   pellets whereof the combustion temperature is under 2200 K, the        combustion speed at 20 MPa is greater than 30 mm/s and the        oxygen balance between −2 and −4%;    -   extruded monolithic blocks whereof the combustion temperature is        under 2200 K, the combustion speed at 20 MPa is greater than 24        mm/s and the oxygen balance between −2 and −4%.

It is now proposed to give some precise details, non-limiting, on theingredients making up the pyrotechnic compositions of the invention andtheir occurrence rate within said compositions.

The at least one organic nitrogen compound, making up the reducingcharge, can especially be selected from guanidine nitrate,nitroguanidine, guanyl urea dinitramide and their mixtures. It consistsadvantageously of guanidine nitrate (GN).

The at least one basic metal nitrate, constitutive of the (first)oxidizing charge can especially be selected from basic copper nitrate,basic zinc nitrate, basic bismuth nitrate and their mixtures. Itconsists advantageously of basic copper nitrate (BCN).

The at least one alkaline metal perchlorate can especially be selectedfrom potassium perchlorate, sodium perchlorate and their mixtures. Itconsists advantageously of potassium perchlorate (KClO₄).

According to a preferred variant, the principal constitutive ingredientsof the compositions of the invention are therefore:

-   -   guanidine nitrate (GN)    -   basic copper nitrate (BCN) and    -   potassium perchlorate (KClO₄).

With respect to the respective quantities of each of said ingredients,there is generally, independently and advantageously in combination:

-   -   said at least one organic nitrogen compound present at the rate        of 45 to 65% by mass;    -   said at least one basic metal nitrate present at the rate of 15        to 35% by mass;    -   said at least one alkaline metal perchlorate (as already        indicated, present at less than 30% by mass. Its beneficial        action on combustion speed thus expresses, with potentialisation        because of the compacting method, appreciably and this without        consequential and harmful elevation of combustion temperature)        present at the rate of 10 to 25% by mass, advantageously from 10        to 20% by mass.

According to a preferred variant, the pyrotechnic compositions of theinvention include:

-   -   from 45 to 65% by mass of guanidine nitrate (GN),    -   from 15 to 35% by mass of basic copper nitrate (BCN),    -   from 10 to 25% by mass of potassium perchlorate (KClO₄).

The pyrotechnic compositions of the invention, in the form of shapedobjects, granules and pellets, consist essentially (at least at 95% bymass, generally at least at 98% by mass), even exclusively (at 100% bymass), of the principal constitutive ingredients (charges) identifiedhereinabove: said at least one organic nitrogen compound, said at leastone basic metal nitrate and said at least one alkaline metalperchlorate. Said ingredients can in fact themselves constitute at 100%the charges of said pyrotechnic compositions (this is generally thecase), or even constitute said pyrotechnic compositions at 100%.However, the presence, within the compositions of the invention, ofother charges in minimal quantities could not be excluded (in any case,the charges identified hereinabove represent at least 95% by mass,generally at least 98% by mass of charges present) and/or that of atleast one additive (fabrication auxiliary type).

The pyrotechnic compositions of the invention, in the form of extrudedmonolithic blocks, include those principal constitutive ingredients(charges) identified hereinabove (plus possibly, other charges inminimal quantities) in a dry gel. This gel, extrudable per se or mixedwith a solvent, entered upstream to allow extrusion. It occurs in anefficient quantity (to allow extrusion), though limited so as not tosubstantially affect the performances of the compositions of theinvention.

The extruded monolithic blocks of the invention generally include nomore than 10% by mass of such a dry gel. They advantageously includefrom 4 to 6% by mass. In their midst, the synergy of the inventiondevelops with the same intensity.

The presence of at least one additive is also not excluded from thiscontext. The principal constitutive ingredients (charges) and dry gelgenerally represent at least 95% by mass, very generally at least 98% bymass (or even 100% by mass) of said compositions.

With respect to the nature of said gel, it is not per se original. Saidgel is generally selected from cellulosic gels, gels obtained fromacrylic elastomers, ethylene-vinyl-acetate copolymers with a highacetate rate (including over 60% by mass of acetate units), polyesterpolymers, and their mixtures. Said gel advantageously consists of asodium carboxymethyl cellulose gel.

According to its second object, the present invention relates to themethod for obtaining pyrotechnic compositions such as describedhereinabove; a method which characteristically comprises dry compactingof powders.

Said method actually comprises:

-   -   dry mixing of powders (charges) consisting for at least 95% by        mass of a pulverulent reducing charge in the form of at least        one organic nitrogen compound, a pulverulent oxidizing charge in        the form of at least one basic metal nitrate and less than 30%        by mass of a second pulverulent oxidizing charge in the form of        at least one alkaline metal perchlorate; and    -   dry roller compacting of the resulting mixture of powders.

Precise details on the nature of the ingredients in question and theirrate of respective presence are specified hereinabove in the presenttext.

The ingredients making up the desired pyrotechnic compositions occur inthe powdered state. Advantageously, said powders have fine granulometry,less than or equal to 40 μm. Said granulometry (value of the mediandiameter) is generally between 3 and 40 μm.

The steps for dry mixing of the powders and dry roller compacting of themixture obtained are executed conventionally. With respect to dry rollercompacting, it has been explained that it is carried out by passing themixture of the powders between two cylinders, the pressure exerted beinggenerally between 10⁸ and 6.10⁸ Pa. “Simple” compacting with twocylinders having non-machined external surfaces or compacting coupledwith forming with cylinders, whereof the external surface of at leastone of the two is machined to present alveoli, is carried out.

It is incidentally noted here that the originality of the invention asclaimed is not based on the originality per se of the method inquestion, but rather the originality of the execution of said methodwith particular mixtures of powders.

The method of the invention can be limited to these two successive stepsof mixing and dry roller compacting, in the context hereinbelow: that ofdirectly obtaining shaped objects, in the hypothesis of carrying outroller compacting coupled with forming (the external surface of at leastone of the compacting cylinders presenting alveoli).

The method of the invention, in addition to said two mixing and(“simple”) dry roller compacting steps, can include:

a) a dry granulation step (the compacted mixture of powders is forcedmechanically by a rotor via a grater acting as a screen whereof the meshgenerally varies from 500 μm to 3 mm). Roller compacting and granulationcan be carried out inside a single device or in two independent devices.The resulting pyrotechnic compositions are then in the form of granules(see above);

b) a dry granulation step (see above) followed by pelleting (drycompression during which the granules undergo pressure generally ofbetween 4.10⁸ and 10⁹ Pa. It is incidentally noted here that feeding thepelleting mould impressions is much more easily carried out withgranules than with starting powders). The resulting pyrotechniccompositions are thus in the form of pellets (see above);

c) a dry granulation step (see above) followed by mixing the granulesobtained with an extrudable binder and extrusion of said mixture. Thepyrotechnic compositions finally obtained are in the form of extrudedmonolithic blocks loaded with granules.

The variants of the method of the invention which include steps b) andc) hereinabove are particularly preferred. Within the scope of said twovariants, the method of the invention includes the steps of (“simple”)roller compacting and dry granulation of the mixture of the startingpowders.

Irrespective of the exact variant of execution of the method of theinvention, it has proven opportune, especially in reference toincreasing the combustion speed of the prepared pyrotechnic composition,to recycle at least in part the fines generated at the compacting stepand/or at that of granulation, when said granulation step is carriedout. Carrying out a fines recycling rate of between 10 and 30% isspecified.

The invention will now be illustrated, in no way limiting, by theexamples hereinbelow. More precisely, examples illustrating two variantsof the invention (pyrotechnic compositions in the form of pellets(example 5) and extruded monolithic blocks (example 7)), are proposed,to be considered in parallel with comparative examples.

The powders (raw materials) utilised have fine granulometry: a mediandiameter of around 20 μm for KClO₄, 4.5 μm for BCN, 10 μm for GN.

Such powders do not flow and cannot therefore per se be used inindustrial pelleting (it is very difficult to fill the pelleting mouldimpressions).

Table I hereinbelow shows formulation examples as well as thethermodynamic and ballistic performances of pellets (of around 2 mm inthickness) obtained by pelleting (performed at 5.10⁸ Pa) of the powdermixtures not pre-compacted.

TABLE I (direct pelleting) Example Example Example Example 1 2 3 4Constituents KClO₄ 23 26 24 14 CuO 17 12 0 0 Guanidine nitrate 55 62 6460 Guanyl urea 5 0 0 0 dinitramide Basic copper nitrate 0 0 12 26Performances 1.88 1.80 1.74 1.82 Theoretical density Pressure exponent0.21 0.16 0.27 0.31 *Vc at 20 MPa (mm/s) 28.3 29.2 33.6 32 **OB (%) −3−3 −2.9 −3.2 T combustion at 20 2250 2260 2258 2126 MPa (K) Gas yield at1000 K 29.2 30.5 32.5 31.6 (mole/kg) Solid residue rate (%) 25.9 23.519.2 21.2 *Vc = combustion speed **OB = oxygen balance

-   -   The composition of example 4, which offers the best compromise        between combustion speed, gas yield and combustion temperature,        was treated, for example 5, by carrying out a roller compacting        method (pressure between the rollers of 4.10⁸ Pa) and dry        granulation (forcing of the compacted material by a rotor        through a grater equivalent to a screen having a mesh of around        1 mm), upstream of pelleting. The resulting granules, on        completion of the roller compacting and granulation step, had        median granulometry of around 500 μm. They were pelletized        (easily, to the extent where there is no longer a flow problem)        in the same conditions as the powders of examples 1 to 4        (pressure of 5.10⁸ Pa).

Table II hereinbelow shows the contribution of the roller compactingmethod on the ballistic performances of the composition.

TABLE II Example 4 Example 5 Constituents KClO₄ 14 14 Guanidine nitrate60 60 Basic copper nitrate 26 26 Method Direct pelleting Pelleting ofgranules of the mixture of (without fines) powders originating from drycompacting Performances 0.31 0.38 Pressure exponent Vc at 20 MPa (mm/s)32 38.2

Applying the roller compacting and dry granulation method to thecomposition according to example 4 causes a rise in combustion speed to20 MPa of the order of 20%. This increased speed is attributed toimproved closeness of the ingredients after passing through thecompacter. The roller compacting phase causes compression and shearingstresses on the mixture, improving the quality of the mixture. Assaysconducted at various pressures on the compacter have confirmed thispoint. To a certain extent, the ballistics of the formulation istherefore adjustable by the pressure applied to the rollers during thecompacting phase.

Also, the roller compacting and granulation phase generates fines (oflow granulometry), which can be reintroduced in the system. Thisreintroduction also generates greater increase in combustion speed,which may reach 40 mm/s to 20 MPa, in the case of the composition ofexample 5 with 20% of recycling fines.

-   -   Pyrotechnic compositions, having the formulation specified in        Table III hereinbelow, were prepared by extrusion using 4% by        mass of sodium carboxymethyl cellulose as a binder. The same        continuous mixing and extrusion method is carried out.

According to example 6, the powders are directly introduced (with thebinder) to the device.

According to example 7, said powders were pre-compacted and granulatedunder the conditions specified hereinabove for example 5 and theresulting granules are introduced (with the binder) to the device.

The performances of the two compositions, prepared in the form ofmonolithic blocks, are also specified in Table III hereinbelow.

TABLE III Example 6 Example 7 Constituents KClO₄ 15 15 Guanidine nitrate49.3 49.3 Basic copper nitrate 29 29 Dry cellulose gel 4 4 Alumina 2.72.7 Method Alimentation with Alimentation with powders granulesPerformances Theoretical 1.88 1.88 density Pressure exponent 0.44 0.42Vc at 20 MPa (mm/s) 24.2 28.8 OB (%) −3.2 −3.2 T combustion at 20 MPa(K) 2078 2078 Gas yield at 1000 K (mole/kg) 28.8 28.8 Rate of solidresidues (%) 27 27

The intervention of the binder is definitely harmful to performance, interms of combustion speed, of the composition (of example 7 relative tothat similar to example 5). All the same, also in this context ofextruded product (as for the pellet product), carrying roller compactingon dry powders results in significantly improving said combustion speed.The gain obtained is of the same order of magnitude, specifically around20%.

1. A pyrotechnic composition generating gases, whereof at least 95% bymass of the charges are constituted by the constitutive ingredientshereinbelow: a reducing charge in the form of at least one organicnitrogen compound; an oxidizing charge in the form of at least one basicmetal nitrate; and less than 30% by mass of a second oxidizing charge inthe form of at least one alkaline metal perchlorate; and which isobtained via a dry roller compacting step of a pulverulent mixtureessentially constituted by said powder ingredients.
 2. The compositionaccording to claim 1, in the form of shaped objects, granules, pelletsor extruded monolithic blocks.
 3. The composition according to claim 1,wherein that said at least one organic nitrogen compound is selectedfrom guanidine nitrate, nitroguanidine, guanyl urea dinitramide andtheir mixtures; in that said at least one organic nitrogen compoundconsists advantageously of guanidine nitrate.
 4. The compositionaccording to claim 1, wherein that said at least one basic metal nitrateis selected from basic copper nitrate, basic zinc nitrate, basic bismuthnitrate and their mixtures; in that said at least one basic metalnitrate consists advantageously of basic copper nitrate.
 5. Thecomposition according to claim 1, wherein that said at least onealkaline metal perchlorate is selected from potassium perchlorate,sodium perchlorate and their mixtures; in that said at least onealkaline metal perchlorate consists advantageously of potassiumperchlorate.
 6. The composition according to claim 1, wherein that saidat least one organic nitrogen compound is present at a rate of 45 to 65%by mass.
 7. The composition according to claim 1, wherein that said atleast one basic metal nitrate is present at the rate of 15 to 35% bymass.
 8. The composition according to claim 1, wherein that said atleast one alkaline metal perchlorate is present at the rate of 10 to 25%by mass.
 9. The composition according to claim 1, wherein that it is inthe form of extruded monolithic blocks and in that said blocks includeup to 10% by mass of a dry gel.
 10. The composition according to claim9, wherein that said gel is selected from cellulosic gels, gels obtainedfrom acrylic elastomers, ethylene-vinyl-acetate copolymers with highacetate rate, polyester polymers and their mixtures.
 11. The compositionaccording to claim 9, wherein that said gel is sodium carboxymethylcellulose gel.
 12. A method for obtaining a pyrotechnic compositiongenerating gases, wherein it comprises: dry mixing of powders consistingfor at least 95% by mass of a pulverulent reducing charge in the form ofat least one organic nitrogen compound, a pulverulent oxidizing chargein the form of at least one basic metal nitrate and less than 30% bymass of a second oxidizing pulverulent charge in the form of at leastone alkaline metal perchlorate; and dry roller compacting of theresulting mixture of powders.
 13. The method according to claim 12,wherein that said powders have granulometry less than or equal to 40 μm.14. The method according to claim 12, wherein that it further comprisesdry granulation of the compacted mixture of powders for generatinggranules.
 15. The method according to claim 14, wherein that it furthercomprises pelleting of said granules.
 16. The method according to claim14, wherein that it further comprises mixing said granules with a geland extruding said gel loaded with said granules.
 17. The methodaccording to claim 12, wherein that it comprises recycling finesgenerated during the dry compacting phase and/or during the granulationphase.
 18. The method according to claim 14, wherein that it comprisesrecycling fines generated during the granulation process.